The present invention relates to a method of securing tubes between tube sheets of a heat exchanger, with the tube sheets being interconnected via a preferably rigid housing. The tubes and/or the tube sheets are made of a steel that has to be annealed, for stress-relieving purposes, after a welding process.
With this type of thermal treatment of heat exchangers having tubes extending between two tube sheets, locally varying temperatures cannot be avoided, so that no matter how carefully the thermal treatment is carried out, stresses remain in the heat exchanger. These stresses affect particularly adversely the large number of weld seams that are provided between the tubes and the tube sheets, and that separate the media that is to be heated up and cooled off in the heat exchanger. Furthermore, it is complicated, time consuming, and expensive to repair the heretofore known constructions.
Up to now, to secure the tubes between tube sheets of heat exchangers, the ends of the tubes were generally welded directly to the tube sheets. As a results, it was necessary, due to the material of the tubes and/or of the tube sheets, which had to be annealed after a welding process, to subject the heat exchanger to a subsequent thermal treatment, at least in the region of the weld seams, so that local stress and hardening from the welding can be removed.
It is furthermore known to provide the outer surface of tube sheets used to produce heat exchangers with a plating. In this case also the weld seams between the tubes and the plating must be subsequently thermally treated if the tubes are to made of a steel that has to be thermally treated after a welding process to remove local stress and hardening. In the last-mentioned case, it is possible in the event of repair to close-off one or more of the tubes by placing into the plating a plug that is made of the plating material or of a material that is equivalent thereto. Since this plug is welded solely to the plating, no subsequent thermal treatment is required in this repair situation. However, the tubes that are closed-off by a plug no longer take part in the heat exchange.
It is an object of the present invention to provide a method of securing tubes between tube sheets of heat exchangers, whereby it is possible to have a stress-free connection between the tubes and the tube sheets, and a simple repair, even if the tubes and/or the tube sheets are made of a steel that must be annealed after a welding process.